opment in a plant but are parts of the normal part of a
plant’s metabolism. They often serve to discourage
being eaten by making the plant taste bad or have a
toxic effect. Examples of secondary compounds are
nicotine and caffeine.
secondary consumer(carnivore) An organism in
one trophic level that eats a primary consumer, such as
a fox eating a hen.
secondary growth Secondary growth occurs in
stems and roots and usually results in the thickening of
the diameter by the addition of vascular tissue. Used
also to describe an area of regrown forest.
secondary immune response The immune response
that follows a second or subsequent encounter with a
particular antigen. Usually the response is more severe
than the PRIMARY IMMUNE RESPONSE.
secondary productivity The rate of new biomass
production that is nutrient material synthesized by con-
sumers over a specific time frame in an ecosystem.
secondary structure Level of structural organization
in proteins described by the folding of the polypeptide
chain into structural MOTIFs such as ALPHA HELIXes
and BETA SHEETs, which involve hydrogen bonding of
backbone atoms. Secondary structure is also formed in
NUCLEIC ACIDS, especially in single-stranded RNAs by
internal BASE PAIRING.
secondary succession Ecological succession in com-
munities that have been disturbed, such as a forest
turned to an agricultural field that then reverts back to
forest; the natural recovery process from disturbance.
second law of thermodynamics States that
entropy, ameasure of disorder, increases in the universe
and is spontaneous. Elements in a closed system will
tend to seek their most probable distribution, and
entropy always increases. It is a measure of unusable
energy and a gauge of randomness or chaos within a
closed system. Also called the Law of Increased
Entropy. Along with the first law of thermodynamics,
these two laws serve as the fundamental principles of
physics.
See alsoFIRST LAW OF THERMODYNAMICS.second messenger An intracellular METABOLITEor
ion increasing or decreasing as a response to the stimu-
lation of RECEPTORs by AGONISTs, considered as the
“first messenger.” This generic term usually does not
prejudge the rank order of intracellular biochemical
events.sedimentary rock A rock composed of sediment
that accumulated over millions of years in the bottom
of water bodies and is turned into rock by pressure
through the process of lithification. Sedimentary
rock identification is primarily based on composition
and can be based on chemistry, fossil or organic
composition, or clastics (small fragments of rock or
mineral).seed Astructure produced by a terrestrial plant in
which the young embryo is encased in a protective cov-
ering and contains stored food.segmentation Being divided into segments, e.g.,
earthworms.selection coefficient A measure of the relative
strength of selection acting against a specific genotype.
It is calculated by subtracting each fitness value from
1.0. It is the cost associated with a given mutation on
an organism’s fitness.selective permeability Refers to the control a cell
membrane has over what can pass through it. The
membrane controls which specific molecules may enter
or leave the cell by using either PASSIVE TRANSPORTor
ACTIVE TRANSPORTor by way of a VESICLE.selective permeability 303